1. Field of the Invention
Hereinafter, the terms extrusion, cryoextrusion, cold extrusion and extruder as an adjective are used indifferently; these terms are perfectly known to those skilled in the art, some also use the expression “mixer” or “screw mixer”.
2. Related Art
The shaping of food products has become a major issue for the industry in recent years. Consumers are increasingly demanding food products in portions, in original and varied shapes, so that they can easily apportion the quantities that are necessary for their meal and that also arouse gustatory desire. Another issue is to provide food professionals with apportioned products that can easily be used.
To meet this requirement, the industry has various techniques available, but which are not, however, fully satisfactory.
The simplest technique is the hollow punch or mold technique. The major drawback in this technique is that it does not make it possible to obtain complex shapes nor does it make it possible to use a liquid raw material, and it is a major source of waste which is difficult to reuse given the increasingly strict food standards.
The technique that is commonly used is the “pellet machine”, consisting of two counter-rotating hollow cylinders between which a food paste, previously deep frozen or not, is poured, which then leaves in the form of “pellets”, which can then be subjected to a freezing step. Such a method is used to make deep frozen food products of the “block” soup, portioned spinach, and other such types. The products obtained in this way are not satisfactory because not only do they have an unpleasing external appearance, requiring the use of opaque packages, but they also have a shape that is limited by the geometry of the machine which makes it possible to produce only pellets, and in no circumstances play shapes for example for children. Finally, it is not possible to accurately apportion the quantity of food product for the “pellets”. The product losses are also significant.
Another technique that can be used is the extrusion of food products, in particular cold extrusion. Many food products can be extruded, whether solid or semi-solid, such as, for example, doughs for bread, savory biscuits or products, starches, vegetables, meats, ice cream, chocolate, soft sweets, chewing gum, fruit jelly, caramel, cereals, vegetable proteins, casein, cheese spread, foods for animals, etc., this list obviously not being exhaustive.
Conventionally, an industrial extruder consists of a long cylinder comprising at least one extrusion screw on the inside, with a feed hopper at one of its ends and an output nozzle at its other end. Generally, the extrusion screw is driven by a rotary motor, the rotation speed of which is controlled by a frequency adjuster.
In low-temperature extrusion, either the products are cooled or even frozen upstream of the extrusion step, as described, for example, in the document U.S. Pat. No. 4,795,650, or the products are directly cooled or even frozen in the extruder itself. In the latter case, either the cooling is done from the outside of the extruder, that is to say that a coolant circulates around the body of the extruder, for example brine, ammonia, glycolated water, liquid nitrogen, carbon dioxide, as described, for example, in US 2003/0211192 or US 2005/0132902; or the cooling is done by direct contact by injecting a coolant into the extruded product, as described, for example, in the documents EP-0 250 381 or US-2006/0283196.
According to one of the known cryoextrusion versions, the circulation means for a coolant consists of a double jacket surrounding the extrusion screw and in which circulates a coolant. The double jacket can consist of a number of independent modules interlinked by ducts of pipe type, enabling the coolant to pass from one module to another, the extrusion screw or screws being positioned in the cylinder formed by the association of the modules of the double jacket, each module being provided with its coolant intake and its gas extraction, and in some cases, some of the modules are connected in gas circulation series, in pairs.
These methods, theoretically attractive, are difficult to exploit industrially. In practice, they have the major drawback of blocking the extruder screw or screws by too rapid a drop in the temperature of the product. In practice, when the temperature of the extruded food product drops too rapidly, the viscosity of the product increases very rapidly and leads to an increase in the torque on the screw or screws and therefore in the motor current, thus triggering the motor's thermal trip and therefore immobilizing said motor. For this reason, this type of cold extrusion method is difficult to use as such in industry to shape food products that have a high viscosity.
Moreover, it should be indicated that controlling the output temperature of the products in practice proves very difficult.
Since the temperature of the product is not easy to control, the output product tends to gradually become too cold, with the consequence of hardening the product and risking jamming the machine, whereas, if the modules are too hot, it is difficult to obtain the desired temperature.
The current situation is characterized by the fact that it is the operator who almost empirically manages the situation: he measures the temperature of the products at the shaping output and imprecisely either undertakes no action or increases the injection of cryogen to try to make up for an insufficiently low output temperature.